1. Field of the Invention
This invention relates to a switching module, a power converter and a power converter composed of using the switching modules, and more particularly to a switching module composed of a plurality of series connected self-turn-off devices, a power converter with low-loss snubber circuits and a power converter with low-loss snubber circuits composed of using the switching modules.
2. Description of the Related Art
A switching module SM0 composed of a single self-turn-off device so far used is shown in FIG. 25. This switching module SM0 is composed of a single self-turn-off device S1, such as an IGBT (Insulated Gate Bipolar Transistor), and a single freewheeling diode D1 which is connected in antiparallel with self-turn-off device S1. A collector terminal (a positive side terminal) of self-turn-off device S1 is led out as a first external terminal 1 and an emitter terminal (a negative side terminal) of self-turn-off device S1 is led out as a second external terminal 2, and further, gate signal terminals 30 for the ON/OFF control of self-turn-off device S1 are led out of switching module SM0.
An example of the configuration of a conventional neutral point clamped power converter (hereinafter referred to as "NPC inverter") for one phase, which is composed of using such switching modules SM0 is shown in FIG. 26. Further, a circuit configuration diagram of the NPC inverter shown in FIG. 26 is illustrated in FIG. 27.
The NPC inverter illustrated in FIGS. 26 and 27 is composed of 4 pieces of the series connected switching modules SM01-SM04, which are respectively composed of a single self-turn-off device S1, S2, S3 and S4 and a single freewheeling diode D1, D2, D3 and D4 connected in antiparallel with them. Switching modules SM01-SM04 are connected in series by connecting external terminal 2 of the positive side switching module to external terminal 1 of the negative side switching module. Further, each of self-turn-off devices S1, S2, S3 and S4 is connected in parallel with a snubber circuit. Each of the snubber circuits is composed of a snubber diode Ds, a snubber capacitor Cs which is series connected to snubber diode Ds and a snubber resistor Rs which is connected in parallel with snubber diode Ds. Numerical codes 1-4 suffixed to the devices indicate corresponding modules SM01-SM04. Between a connecting point of switching modules SM01 and SM02 and a connecting point of switching modules SM03 and SM04, clamp diodes Dc1 and Dc2 are connected in series in the direction reverse to the polarity of self-turn-off devices S1-S4. From a DC voltage source (Voltage Vd=Vd1+Vd2) composed of capacitors Cp1 and Cp2 with voltages Vd1 and Vd2, a positive side terminal 10, a zero-voltage terminal 11 and a negative side terminal 12 are led out. Series connected four pieces of switching modules SM01-SM04 are connected between positive and negative side terminals 10 and 12 through line inductances L1, L3. Further, the connecting point of clamp diodes Dc1, Dc2 is connected to zero-voltage terminal 11 and a line inductance L2 is also shown here. From the connecting point of both switching modules SM02 and SM03, an output terminal 20 of NPC inverter is led out.
Next, the operation of the NPC inverter shown in FIGS. 26 and 27 will be described.
One example of the relationship of the switching operation and voltage levels of the self-turn-off devices S1-S4 is shown below. This NPC inverter outputs voltage Vd1 when self-turn-off devices S1 and S2 are ON, outputs zero voltage when self-turn-off devices S2 and S3 are ON, and outputs voltage -Vd2 when self-turn-off devices S3 and S4 are ON. For making the description simple, it is assumed that Vd1=Vd2=Vd/2.
In the NPC inverter, for instance, if self-turn-off devices S1-S3 are turned ON simultaneously, a short-circuit of DC voltage Vd1 is formed in the route of self-turn-off devices S1-S2-S3 and clamp diode Dc2, and excessive short-circuit current flows through devices in the short-circuit. To prevent this short-circuit current, self-turn-off devices S1 and S3 are reversely operated (when one of them is ON, the other is OFF) and the self-turn-off devices S2 and S4 are also reversely operated.
Next, the operation of the snubber circuits shown in FIGS. 26 and 27 will be described. Each of the snubber circuit is arranged close to respective self-turn-off devices S1-S4 to reduce the influence of the line inductance. If self-turn-off device S1 is turned OFF in the state wherein current is flowing through line inductance L1 and self-turn-off devices S1 and S2, the residual energy of line inductance L1 charges snubber capacitor Cs1 via snubber diode Ds1 as shown in FIG. 28. The voltage of capacitor Cs1 becomes the sum of DC voltage Vd1 and the voltage by the residual energy of line inductance L1. The charge in snubber capacitor Cs1 is discharged through the route of snubber capacitor Cs1.fwdarw.snubber resistor Rs1.fwdarw.self-turn-off device S1 when self-turn-off device S1 is next turned ON and the charge in snubber capacitor Cs1 drops nearly to zero. This also applies to other self-turn-off devices S2-S4 as shown in FIG. 29.
In switching module SM0 illustrated in FIG. 25, the wiring length between self-turn-off device S1 and freewheeling diode D1 connected in antiparallel with it becomes short and the line inductance between them can be reduced. But the inductance of wires required between switching module SM0 and other devices cannot be reduced. Further, in case of the snubber circuit in the circuit configuration shown in FIG. 27, the snubber energy is all consumed by snubber resistors Rs1-Rs4 and therefore, its efficiency becomes worse.
In an effort to solve this defect, a low-loss snubber circuit for NPC inverter is proposed (1995 National Convention of the Institute of Electrical Engineers of Japan, Report, No. 5, p. 320, 1178: "Clamp-Snubber for 3-Level-Inverter") This proposed snubber circuit is shown in FIG. 30.
FIG. 30 shows one example of a main circuit configuration for a single phase of an NPC inverter using such low-loss snubber circuits. Further, for the circuit shown in FIG. 30, an example of a main circuit configuration for a single phase of an NPC inverter applied with conventional switching modules SM0 shown in FIG. 25 is shown in FIG. 31.
In FIGS. 30 and 31, snubber diodes Ds1-Ds4, Ds22 and Ds32, snubber capacitors Cs1-Cs4 and snubber resistors Rs1-Rs4 were added as snubber circuit elements for the discharging snubber circuits of the NPC inverter shown in FIGS. 26 and 27.
The operation of the NPC inverter using low-loss snubber circuits shown in FIGS. 30 and 31 will be described. When self-turn-off device S1 is turned OFF in the state wherein the current is flowing via line inductance L1 and self-turn-off devices S1 and S2, the voltage of self-turn-off device S1 rises by the residual energy of line inductance L1. When the voltage of self-turn-off device S1 exceeds the voltage of snubber capacitor Cs1, a forward voltage is applied to snubber diode Ds1 and snubber diode Ds1 becomes the ON state. As a result, the residual energy of line inductance L1 flows into snubber capacitor Cs1. At this time, if the voltage of snubber capacitor Cs1 rises higher than DC voltage Vd1, excess voltage is discharged by snubber resistor Rs1 so that the voltage of snubber capacitor Cs1 becomes equal to voltage Vd1.
These states are shown in FIG. 32 and FIG. 33. The voltage of snubber capacitor Cs1 is applied to self-turn-off device S1, and then DC voltage Vd1 is steadily applied thereto. When self-turn-off device S1 is turned ON, capacitor Cs1 does not discharge and is kept being clamped at DC voltage Vd1. Therefore, excess voltage only at the time of turn-OFF is discharged through snubber resistor Rs1 and thus, a low-loss snubber circuit can be achieved.
Next, the operation when self-turn-off device S2 is turned OFF will be described. When self-turn-off device S2 is turned OFF in the state wherein self-turn-off device S2 is in the ON state and the current is flowing through line inductance L2, clamp diode Dc1 and self-turn-off device S2, the voltage of self-turn-off device S2 is raised by the residual energy of line inductance L2. If terminal voltage of self-turn-off device S2 exceeds the voltage of snubber capacitor Cs2, snubber diode Ds2 becomes the ON state and the residual energy of line inductance L2 flows into snubber capacitor Cs2. As a result, the voltage of snubber capacitor Cs2 rises, and snubber capacitor Cs2 is kept charged as there is no place for the charge to go even when the voltage of snubber capacitor Cs2 becomes higher than DC voltage Vd2. The circuit diagram in this state is shown in FIG. 34.
FIG. 35 shows the route for discharging overcharged charge of snubber capacitor Cs2. When self-turn-off device S2 is turned ON next, self-turn-off device S3 is also in the ON state according to the switching control described above. The discharging route is in the order of snubber capacitor Cs2.fwdarw.self-turn-off device S2.fwdarw.self-turn-off device S3.fwdarw.clamp diode Dc2.fwdarw.DC voltage source Cp2.fwdarw.snubber diode Ds22.fwdarw.snubber resistor Rs2. The voltage of snubber capacitor Cs2 is clamped at voltage Vd2 and only voltage in excess of voltage Vd2 is discharged via snubber resistor Rs2. This is also the same in the snubber circuits of self-turn-off devices S3 and S4.
For the conventional low-loss snubber circuits shown in FIGS. 30 and 31, snubber diodes Ds22 and Ds32 become newly required. The operation of these diodes Ds22 and Ds32 will be described in the following. For instance, when self-turn-off devices S1 and S2 are in the ON state, the potential at the positive side terminal of self-turn-off device S2, that is, at one end of snubber capacitor Cs2 becomes equal to the potential at positive side terminal 10 of the DC voltage source. When assuming that there is no snubber diode Ds22, the potential at the other end of snubber capacitor Cs2 becomes equal to the potential at negative side terminal 12 of the DC voltage source. That is, snubber diode Ds22 is needed to prevent the state that total voltage of the DC voltage source is applied to snubber capacitor Cs2, that is, total voltage of the DC voltage source is applied to self-turn-off device S2. In the same manner, snubber diode Ds32 prevents application of total voltage of the DC power source to self-turn-off device S3.
In the circuit configuration of the NPC inverter using conventional low-loss snubber circuits shown in FIGS. 30 and 31, there is such a problem that the combination of the self-turn-off devices in the ON/OFF switching must be self-turn-off devices S1 and S2, self-turn-off devices S2 and S3, and self-turn-off devices S3 and S4. Here, there is an example of other switching control system proposed in the Japanese Patent Disclosure (Kokai) No. Hei, 4-295279.
According to this control system, it is possible to reduce loss by eliminating useless switching operation by turning ON self-turn-off devices only which is required depending on the output current direction. That is, when the output current is positive, self-turn-off devices S1 and S2 are ON and the output voltage level is Vd1 (Vd/2);
when the output current is positive, self-turn-off device S2 is ON and the voltage level is 0 volts; PA1 when the output current is negative, self-turn-off device S3 is ON and the voltage level is 0 volts; and PA1 when the output current is negative, self-turn-off devices S3 and S4 are ON and the voltage level is -Vd2 (=-Vd/2).
In other words, when the output current is positive, self-turn-off devices S3 and S4 are kept OFF so that the useless switching thereof is not carried out. Further, when the output current is negative, self-turn-off devices S1 and S2 are kept OFF and no useless switching thereof is carried out. Thus, the switching loss can be reduced.
However, when it is tried to apply this control system to the NPC inverter using convention low-loss snubber circuits shown in FIGS. 30 and 31, a problem described below will arise.
That is, for instance, when the output current is positive, it may be needed to turn ON/OFF self-turn-off device S2 while self-turn-off devices S3 and S4 are kept in the OFF state. In this case, even when self-turn-off device S2 is turned ON, as self-turn-off device S3 is kept in the OFF state excess voltage of snubber capacitor Cs2 is not discharged. Therefore, the voltage of snubber capacitor Cs2 rises every time when self-turn-off device S2 is turned OFF, and finally, snubber capacitor Cs2 is charged up to the total voltage (Vd=Vd1+Vd2) of the DC voltage source. As a result, the voltage of self-turn-off device S2 becomes overvoltage. It is therefore difficult to apply the control system described above to the NPC inverter having low-loss snubber circuits shown in FIGS. 30 and 31.
The conventional switching module and the conventional power converter described above have the following problems.
1. When a power converter is composed using conventional switching modules, external wirings become long, line inductances increase and as a result, troubles in the circuit operation are caused. PA0 2. To reduce an influence of line inductances, it is required to arrange a snubber circuit as close to a switching module as possible, and therefore, the snubber circuit configuration is restricted. PA0 3. A conventional snubber circuit installed close to a switching module has a large loss and the efficiency of a power converter becomes worse. In connection with this, a cooling equipment inevitably becomes large in size. PA0 4. A neutral point clamped power converter equipped with a conventional low-loss snubber circuit has a restriction for switching control and self-turn-off devices may be subject to application of excessive voltage depending on a control system.